1. Field of the Invention
The present invention relates to a method for producing core-shell structured resin particles. The present invention further relates to core-shell structured toner for developing electrostatic latent images, which contains the core-shell structured resin particles.
2. Description of the Related Art
It is known that core-shell structured particles which is formed of a core and a shell can contain materials having different physical properties in its core and shell, respectively, and can provide various characteristics. Because of these various characteristics, studies for application of core-shell structured particles have been made broadly. For example, studies have been made regarding pharmaceutical products, cosmetic products, electronic materials, foods, adhesives and constructional materials.
Also, in a field of recording material, studies for application of core-shell structured particles are under progress. Recently, for the image formation on the basis of the electrophotographic system, efforts are made to lower the fixation temperature of toner in accordance with greater demand for saving energy. As one example, a suggestion has been made to lower the fixation temperature by using a resin having low softening temperature. However, when the resin having low softening temperature is used for toner without any modification, blocking may occur in a static state, during, for example, storage or transportation of the toner. For such reasons, a suggestion has been made to use core-shell structured toner in which a resin having low softening point is coated with a resin having high softening temperature (Japanese Patent Application Laid-Open Nos. 2006-276073 and 2007-3840, and Japanese Patent Publication No. 4135654).
Meanwhile, even when a core-shell structure is successfully formed, under a highly humid environment, moisture may penetrate into the core to cause a change in physical properties of the core. Thus, in order to avoid the influence of moisture, it can be considered to use hydrophobic resin microparticles as a core and produce core-shell structured toner. However, according to the methods described in Japanese Patent Application Laid-Open Nos. 2006-276073 and 2007-3840, and Japanese Patent Publication No. 4135654, the core particles are prepared in an aqueous medium, and thereby, the core particles exhibit hydrophilicity at a certain level.
According to a kneading pulverization method, resin microparticles are produced in vapor phase, and therefore the resin microparticles having high hydrophobicity can be produced. Moreover, efforts have been made to develop a method for producing core-shell structured toner by dispersing resin microparticles having high hydrophobicity, which have been prepared by a kneading pulverization method, into an aqueous medium (Japanese Patent Application Laid-Open Nos. H9-34167 and 2000-112175).
However, the methods described in Japanese Patent Application Laid-Open Nos. H9-34167 and 2000-112175 use an organic solvent for forming a core-shell structure, and thus a problem remains in that a huge burden on an environment is unavoidable during producing a toner.